Process for purification of alkyl alkanoate

ABSTRACT

A process for the recovery of substantially pure alkyl alkanoate, such as ethyl acetate, from an impure feedstock. The impure feedstock is contacted with a selective hydrogenation catalyst in the presence of hydrogen in a selective hydrogenation zone maintained under selective hydrogenation conditions effective for selective hydrogenation of impurities containing reactive carbonyl groups thereby to hydrogenate the impurities to the corresponding alcohols. After recovery from the selective hydrogenation zone of a selectively hydrogenated reaction product mixture including the alkyl alkanoate and the corresponding alcohols, this is distilled in one or more distillation zones so as to produce substantially pure alkyl alkanoate therefrom which is recovered.

This invention relates to a process for purification of an impurefeedstock containing an alkyl alkanoate which contains up to 12 carbonatoms as well as at least one impurity selected from an aldehyde and aketone and containing the same number of carbon atoms as the alkylalkanoate.

Alkyl alkanoates can be produced by esterification of an alkaroic acidwith an alkanol. An example is the esterification of acetic acid withethanol according to equation (1):

CH₃.CO.OH+CH₃CH₂OH=CH₃.CO.O.CH₂.CH₃+H₂O  (1)

Because the esterification reaction does not tend to lead to formationof by-products which have boiling points close to that of the alkylalkanoate, recovery of substantially pure alkyl alkanoate from theesterification product mixture is usually not complicated by thepresence of by-products of the esterification reaction.

Alkyl alkanoates can alternatively be produced using the Tischenkoreaction. For example ethyl acetate can be produced from acetaldehydeaccording to the Tischenko reaction given in equation (2):

2CH₃.CHO=CH₃.CO.O.CH₂.CH₃  (2).

It is also possible to produce alkyl alkanoates from alkanols bydehydrogenation. For example ethyl acetate can be made from ethanol bydehydrogenation according to equation (3):

2CH₃.CH₂.OH=CH₃.CO.O.CH₂.CH₃+2H₂  (3).

Catalytic dehydrogenation of alcohols with reduced copper under ultraviolet light was described by S. Nakamura et al, in Bulletin of theChemical Society of Japan (1971), Vol. 44, pages 1072 to 1078.

K. Takeshita et al described reduced copper catalysed conversion ofprimary alcohols into esters and ketones in Bulletin of the ChemricalSociety of Japan, (1978) Vol. 51(9), pages 2622 to 2627. These authorsmention that the mechanism for ester formation has been described in theliterature as the Tischenko reaction. That is to say thatdehydrogenation of ethanol yields acetaldehyde as an intermediate whichcombines according to the Tischenko reaction to produce ethyl acetate.Alternatively, or as well, 1 mole of ethanol may combine with 1 mole ofacetaldehyde to yield 1 mole of ethyl acetate and 1 mole of hydrogenaccording to equation (4):

CH₃CH₂OH+CH₃.CHO=CH₃.CO.O.CH₂.CH₃+H₂  (4).

Production of esters from primary alcohols by dehydrogenation usingbromous acid or a salt thereof in acid medium is described inJP-A-59/025334.

In SU-A-362814 there is described a process for production of ethylacetate by dehydrogenation of ethanol at 180° C. to 300° C. in thepresence of a copper catalyst containing zinc as an activator with anethanol feed rate of 250 to 700 liters per liter of catalyst per hour.

The dehydrogenation of ethanol to form ethyl acetate is described inGB-A-287846. This proposes use of a dehydrogenating agent, such as acopper catalyst, a temperature of from 250° C. to 500° C., and apressure of more than 10 atmospheres (1.013×10⁶ Pa)

Vapour phase contact of ethanol at a temperature above its criticaltemperature with a catalyst comprising copper and a difficultlyreducible oxide, such as zinc oxide or manganese oxide, is proposed inGB-A-312345 for the production of ethyl acetate, use of a temperature of375° C. and a pressure of 4000 psi (27.58 Mpa) being suggested.

GB-A-470773 teaches a process for conversion of ethanol to ethyl acetateby dehydrogenating ethanol over a catalyst consisting of a reducedmetal, for example, copper on infusorial earth with 10% uranium oxide aspromoter, maintained at a temperature of 220° C. to 260° C. , removingby condensation some of the gas-vapour product rich in hydrogenresulting from the reaction, and returning the gaseous remainder rich inhydrogen to the catalysing zone.

EP-A-0151886 describes a process for the preparation of C₂₊ esters ofalkyl carboxylic acids from C₂₊ primary alcohols which comprisescontacting a vaporous mixture containing a primary C₂₊ alkanol andhydrogen in an alkanol:hydrogen molar ratio of from 1:10 to about 1000:1at a combined partial pressure of alkanol and hydrogen of from about 0.1bar (10³ Pa) up to about 40 bar (4×10⁶ Pa) and at a temperature in therange of from about 180° C. to about 300° C. in a catalytic reactionzone with a catalyst consisting essentially of a reduced mixture orcopper oxide and zinc oxide, and recovering a reaction product mixturecontaining a primary C₂₊ alkyl ester of an alkyl carboxylic acid whichester contains twice as many carbon atoms as the primary C₂₊ alkanol.

In EP-A-0201105 there is described a method for converting primaryalcohols, such as ethanol, to their corresponding alkanoate esters whichinvolves the regulation of the mole feed ratio of hydrogen gas toalkanol in the reaction zone of a copper chromite containing catalyst.

Separation of ethyl acetate from a composition comprising ethyl acetate,ethanol and water is disclosed in JP-A-05/186392 by feeding thecomposition to a distillation column to obtain a quasi-azeotropicmixture comprising ethyl acetate, ethanol and water, condensing it,separating the condensate into an organic layer and an aqueous layer,returning the organic layer to the column, and recovering ethyl acetateas a bottom product from the column.

EP-A-0331021 describes how carbonylation of olefins to producemonocarboxylate esters causes formation of aldehydes and acetals asbyproducts. Monocarboxylate esters produced in this way are subjected toa three step purification process involving treatment with a stronglyacidic agent, followed by hydrogenation and distillation. The initialtreatment with a strongly acidic agent is intended to convert acetals tovinyl ethers and aldehydes and acetals to aldols. The subsequenthydrogenation step then converts these compounds to byproducts which aremore easily separated from the desired monocarboxylate ester.

EP-A-0101910 contains a similar disclosure regarding carbonylation ofolefins to give monocarboxylate esters. It proposes treatment of themonocarboxylate ester with hydrogen at elevated temperature in thepresence of an acidic ion exchanger or zeolite doped with one or moremetals of Group VIII of the Periodic Table, followed by hydrogenation.It is stated that acetals present as byproducts are converted to vinylethers which are converted by hydrogenation to low boiling esters or thealdehydes and acetals are converted to high boilers by an aldolreaction. Unsaturated ketones are converted to saturated ketones.

One particular problem in production of alkyl alkanoates bydehydrogenation of an alkanol is that the reaction product mixture tendsto be a complex mixture including esters, alcohols, aldehydes andketones. The reaction product mixtures contain components with boilingpoints close to that of the desired alkyl alkanoate or alkanoates. Insome cases such components can form azeotropes, including azeotropeswith the desired alkyl alkanoate or alkanoates whose boiling points areclose to that of the alkyl alkanoate or alkanoates. This is a particularproblem when a high purity alkyl alkanoate, such as ethyl acetate, isdesired.

The present invention accordingly seeks to provide a novel process forrecovery of a substantially pure alkyl alkanoate from an impurefeedstock, for example a crude product produced by dehydrogenation of analkanol which contains by-products whose boiling point is close to thatof the desired alkyl alkanoate or alkanoates and which, in some cases atleast, from azeotropes with the alkyl alkanoate or alkanoates whoseboiling points are close to that of the desired alkyl alkanoate oralkanoates. It further seeks to provide a process for purification of animpure feedstock containing an alkyl alkanoate containing up to 12carbon atoms which further contains as an impurity at least one aldehydeand/or ketone which contains the same number of carbon atoms as thealkyl alkanoate so as to result in production of a substantially purealkyl alkanoate product. In addition the invention seeks to provide animproved process for the production of an alkyl alkanoate bydehydrogenation or oxidation of an alkanol, by reaction of an alkanolwith an alkanal, or by oxidation of an alkanol to an alkanal followed bythe Tischenko reaction which enables production of a substantially purealkyl alkanoate product, despite the presence in the crude reactionproduct of aldehydes and ketones which would otherwise contaminate thealkyl alkanoate product.

According to the present invention there is provided a process for thepurification of an impure feedstock comprising an alkyl alkanoate whichcontains up to 12 carbon atoms which comprises:

(a) providing an impure feedstock containing an alkyl alkanoate whichcontains up to 12 carbon atoms, said feedstock further containing atleast one impurity which is selected from an aldehyde and a ketone andwhich contains the same number of carbon atoms as said alkyl alkanoate;

(b) contacting said impure feedstock with a selective hydrogenationcatalyst in the presence of hydrogen in a selective hydrogenation zonemaintained under selective hydrogenation conditions effective forselective hydrogenation of impurities containing reactive carbonylgroups thereby to hydrogenate said impurities to the correspondingalcohols;

(c) recovering from the selective hydrogenation zone a selectivelyhydrogenated reaction product mixture comprising said alkyl alkanoate,hydrogen, and said corresponding alcohols;

(d) distilling material of the selectively hydrogenated reaction productmixture in one or more distillation zones so as to produce substantiallypure alkyl alkanoate therefrom; and

(e) recovering said substantially pure alkyl alkanoate.

The invention further provides a process for the production of an alkylakanoate containing up to 12 carbon atoms by dehydrogenation of analkanol which comprises: (i) contacting a vaporous mixture containing an

alkanol and hydrogen with a dehydrogenation catalyst in adehydrogenation zone maintained under dehydrogenation conditionseffective for dehydrogenation of an alkanol to yield an alkyl alkanoatecontaining up to 12 carbon atoms;

(ii) recovering from the dehydrogenation zone an intermediate reactionmixture comprising hydrogen and liquefiable products comprising saidalkyl alkanoate, said alkanol, hydrogen and by-products containingreactive carbonyl groups; and

(iii) subjecting at least a portion of the liquefiable products of theintermediate reaction product mixture as impure feedstock to a processas outlined in the preceding paragraph.

The impure feedstock may be effectively any feedstock which contains analkyl alkanoate, such as ethyl acetate, or a mixture of alkylalkanoates, possibly water, an alkanol, such as ethanol, or a mixture ofalkanols, and minor amounts of impurities including aldehydes and/orketones. In the case of ethyl acetate such aldehydes and ketones includen-butyraldehyde, acetone and butan-2-one. Example of such an impurefeedstock are the intermediate reaction product mixtures obtained bydehydrogenation of an alkanol, such as ethanol, or of a mixture ofalkanols, such as ethanol and iso-butanol.

A range of undesirable impurities may be present in the feedstock, someof which would cause separation problems if the feedstock were to bedirectly refined because their boiling points are close to that of thealkyl alkanoate or because, in some cases at leas, they form azeotropeswith the alkyl alkanoate whose boiling point is close to that of thealkyl alkanoate. For example, purification of the specified exemplaryalkyl alkanoates can be complicated by the presence of the impuritiesset out in the following Table 1, the same impurities generally givingrise to problems with all alkyl alkanoates with the same number ofcarbon atoms.

TABLE 1 No. of C atoms Alkyl alkanoate b.p. (° C.) Impurity b.p. (° C.)2 Methyl formate 31.5 Acetaldehyde 20 Propionaldehyde 48 3 Ethyl formate54.5 Propionaldehyde 48 Methyl acetate 56.2 Acetone 56 4 Ethyl acetate77 n-butyraldehyde 75 Methyl propionate 79 Butan-2-one 80 n-propylformate 81.3 5 Methyl butyrate 103.6 n-valeraldehyde 103 Ethylpropionate 100 Pentan-2-one 102 n-propyl acetate 101.6 Pentan-3-one 102n-butyl formate 107.5 6 Methyl valerate 127 n-hexanal 128 Ethyl butyrate123 Hexan-2-one 128 n-propyl propionate 123 Hexan-3-one 124 n-butylacetate 126 n-pentyl formate 132 7 Methyl caproate 150 n-heptanal 152Ethyl valerate 146 Heptan-2-one 151 n-propyl butyrate 145 Heptan-3-one147 n-butyl propionate 146 Heptan-4-one 144 n-pentyl acetate 149 n-hexylformate 156

It will be appreciated by those skilled in the art that Table 1 listsonly some of the possible alkyl alkanoates whose production is embracedwithin the teachings of the present invention. For example, isomericalkyl alkanoates derived from alkanols and/or alkanoic acids withbranched chains can also be mentioned.

Preferably the alkyl alkanoate is a C₂ to C₄ alkyl ester of a C₂ to C₄alkanoic acid, for example, ethyl acetate, n-propyl propionate, orn-butyl butyrate.

For convenience the process will hereafter be described in relation toDurification of impure ethyl acetate feedstocks.

In the case of an impure feedstock resulting from dehydrogenation ofethanol, the ethanol feedstock may contain impurities or impurities maybe formed as by-products in the production of the alkyl alkanoate, forexample, in the course of the dehydrogenation step. Problematicalimpurities are aldehydes and ketones, such as n-butyraldehyde andbutan-2-one in the case of ethyl acetate. In order to minimise problemsdue to the presence of such impurities in the distillation step (d) evenin amounts as small as about 0.1 mol % or less, e.g. about 0.01 mol % orless, problematical impurities are substantially removed as a result ofthe selective hydrogenation step (b). Accordingly, the impure feedstockis contacted in admixture with hydrogen in step (b) with a selectivehydrogenation catalyst. The catalyst type and reaction conditions arechosen so that aldehydes and ketones are hydrogenated to theirrespective alcohols, while hydrogenation of the alkyl alkanoate, e.g.ethyl acetate, is minimal. Among aldehyde and ketone impurities whichmay be present in an impure ethyl acetate feedstock, butan-2-one andn-butyraldehyde, in particular, would otherwise cause problems in anysubsequent distillation. These compounds are hydrogenated in theselective hydrogenation zone in step (b) to the corresponding alcohols,i.e. 2-butanol and n-butanol respectively, which can be readilyseparated from ethyl acetate by distillation.

The mixture supplied to the selective hydrogenation zone in step (b)contains, in addition to ethanol, hydrogen either alone or in admixturewith one or more inert gases that are inert to the reactants andcatalysts in the selective hydrogenation step (b) of the process of theinvention. Examples of such inert gases are nitrogen, methane, andargon. The source of the hydrogen used in the selective hydrogenationstep (b) may be hydrogen formed in the dehydrogenation step andaccordingly may include gas recycled from the downstream end of theselective hydrogenation zone as described further below.

The selective hydrogenation step (b) is typically conducted at atemperature of from about 40° C. to about 120° C., preferably at atemperature in the range of from about 60° C. to about 80° C.

Typical selective hydrogenation conditions include use of afeedstock:hydrogen molar ratio of from about 1000:1 to about 5:1, forexample about 20:1.

The combined partial pressure of feedstock and hydrogen in the selectivehydrogenation zone typically lies in the range of from about 5 bar(5×10⁵ Pa) up to about 80 bar (8×10⁶ Pa), and is even more typicallyabout 24 bar (2.5×10⁶ Pa) to about 50 bar (5×10⁶ Pa)

The selective hydrogenation catalyst used in step (b) of the process ofthe invention is selected to have good activity for hydrogenation ofreactive carbonyl containing compounds, but relatively poor esterhydrogenation activity. Suitable catalysts comprise metals selected fromnickel, palladium and platinum. Ruthenium, supported on carbon, aluminaor silica is also effective, as are other metal catalysts such asrhodium and rhenium. Preferred catalysts include nickel on alumina orsilica and ruthenium on carbon. Particularly preferred catalysts include5% ruthenium on carbon available from Engelhard.

The rate of supply of impure feedstock to the selective hydrogenationzone typically corresponds to a liquid hourly space velocity (LHSV) offrom about 0.1 hr⁻¹ to about 2.0 hr⁻¹, preferably from about 0.2 hr³¹ ¹to about 1.5 hr⁻¹. When using a nickel containing catalyst the LHSV maybe, for example, from about 0.3 hr⁻¹ to about 0.5 hr⁻¹.

Step (c) of the process of the present invention comprises recoveringfrom the selective hydrogenation zone a selectively hydrogenatedreaction product mixture comprising alkyl alkanoate (e.g. ethylacetate), alkanol (e.g. ethanol), hydrogen and hydrogenated impurities.Typically this includes a condensation stem in order to separateliquefiable materials from a gaseous stream containing unreactedhydrogen which can be recycled for dehydrogenation or for selectivehydrogenation.

The impure feedstock typically contains water and alkanol (e.g. ethanol)in addition to alkyl alkanoate (e.g. ethyl acetate). In this case step(d) of the process of the invention comprises distilling material of theselectively hydrogenated reaction product mixture in one or moredistillation zones. When the alkyl alkanoate is ethyl acetate,distillation is effected so as to produce a first composition comprisingsubstantially pure ethyl acetate and a second composition comprisingethanol and water. In this step the selectively hydrogenated reactionproduct mixture subjected to distillation typically has a water contentof less than about 20 mol %, more usually not more than about 15 mol %.

Ethanol, water and ethyl acetate form a minimum boiling ternaryazeotrope upon distillation thereof.

One method of separating ethyl acetate from ethanol and water involvesextractive distillation with an extractive agent comprising polyethyleneglycol and dipropylene glycol, diethylene glycol, or triethylene glycolas described in U.S. Pat. No. 4569726 or with an extractive agentcontaining dimethyl sulfoxide as described in U.S. Pat. No. 4379028.Hence step (d) may comprise an extractive distillation procedure.

Preferably, however, distillation is carried out in step (d) by aprocedure which takes advantage of the fact that the composition of theminimum boiling ternary azeotrope formed by ethanol, water and ethylacetate depends upon the pressure at which distillation is effected.Hence a preferred distillation procedure comprises supplying material ofthe selectively hydrogenated reaction product mixture to a firstdistillation zone maintained under distillation conditions effective fordistillation therefrom of a first distillate comprising ethyl acetate,ethanol, and water, recovering a first distillate comprising ethylacetate, ethanol, and water from the first distillation zone and abottom product comprising ethanol and water, supplying material of thefirst distillate to a second distillation zone maintained underdistillation conditions effective for distillation therefrom of a seconddistillate comprising ethanol, water, and ethyl acetate (preferably aminor amount of ethyl acetate) and so as to yield a substantially pureethyl acetate bottom product, and recovering a substantially pure ethylacetate bottom product from the second distillation zone. The firstdistillation zone is preferably operated at a pressure less than about 4bar (4×10⁵ Pa), preferably from about 1 bar (10⁵ Pa) up to about 2 bar(2×10⁵ Pa), while the second distillation zone is operated at a higherpressure than that of the first distillation zone, for example at apressure of from about 4 bar (4×10⁵ Pa) to about 25 bar (2.5×10⁶ Pa),preferably from about 9 bar (9×10⁵ Pa) to about bar (15×10⁵ Pa).

It can be shown that in this Dreferred distillation procedure the rateof flow of the first distillate from the first distillation zone to thesecond distillation zone and the corresponding flow rate from the seconddistillation zone to the first distillation zone of the seconddistillate can be minimised by operating one of the distillation zonesso that the distillate has a composition very close to that of theternary azeotrope at that pressure. However, in order to operate thatzone so that the distillate has a composition close to that of theternary azeotrope at its pressure of operation, a high degree ofseparation is required which necessitates use of a distillation columnwith many distillation trays and a high heat input. In addition, sincewater has the highest latent heat of vaporisation out of the threecomponents of the ternary azeotrope, the total heat input to the twozones can be minimised by minimising the water content of the feeds tothe distillation zones.

In addition to forming a ternary azeotrope, the three components of theternary azeotrope can each also form binary azeotropes with one of theother components. For example, ethanol forms a binary azeotrope withwater and also with ethyl acetate it is preferred to select a pressureof operation of the second distillation zone so that the binaryazeotrope between ethanol and ethyl acetate at that pressure has a lowerethyl acetate content than the ternary azeotrope at that pressure andfurther to select a pressure of operation for the first distillationzone and to adjust the flow rates of the distillates between the firstand second zones so that the first distillate has as low a water contentas possible. In this way the second distillate recovered from the seconddistillation zone low content of ethyl acetate.

In the preferred distillation procedure an ethanol rich streamcontaining substantially all of the water in the selectivelyhydrogenated reaction mixture is recovered from the bottom of the firstdistillation zone, while an overhead stream that contains “light”components present in the selectively hydrogenated reaction productmixture is recovered from the first distillation zone, and the firstdistillate comprises a liquid draw stream which is recovered from anupper region of the first distillation zone and which comprises ethylacetate, ethanol, water and minor amounts of other components. By theterm “light” components is meant components that have lower boilingpoints than ethyl acetate and its azeotropes with water and ethanol. Theliquid draw stream typically contains less than about mol 10% water. Forexample, it suitably comprises from about 1 mol % to about 6 mol %water, from about 40 mol % to about 55 mol % ethyl acetate, not morethan about 2 mol a minor products (preferably not more than about 1 mol% minor products) and the balance ethanol. Thus it may typically containabout 45 mol % ethyl acetate, about 50 mol % ethanol, about 4 mol %water and about 1 mol % other components. This liquid draw stream ispassed to the second distillation zone. The second distillate, with atypical composition of about 25 mol % ethyl acetate, about 68 mol %ethanol, about 6 mol % water, and about 1 mol % of other components, isrecovered as an overhead stream from the second distillation zone, whilea bottom product comprising ethyl acetate is recovered from the seconddistillation zone which typically contains from about 99.8 mol % toabout 99.95 mol % ethyl acetate; this second distillate is returned tothe first distillation zone, preferably at a point above the feed pointof the liquefiable products of the selectively hydrogenated reactionproduct mixture.

The overhead stream from the first distillation zone contains “light”components present in the intermediate reaction product mixture, such asdiethyl ether, acetaldehyde and acetone. It can be burnt as a fuel.

In this preferred process of the invention the ethanol rich streamrecovered from the bottom of the first distillation zone can, ifdesired, be subjected to treatment for the removal of water therefromthereby to produce a relatively dry ethanol stream which can be used fora purpose which will be described below, if desired. products, includingunknown products, with high boiling points compared to those of ethanoland ethyl acetate. These can be separated from the ethanol and water bydistillation, if desired, prior to effecting removal of water from theresulting distillate. The resulting ethanol stream, after water removal,can be recycled for production of further ethyl acetate.

One suitable method for removal of water from the ethanol rich stream orfrom the distillate resulting from “heavies” removal is molecular sieveadsorption. Azeotropic distillation with a suitable entrainment agent,such as benzene or cyclohexane, can alternatively be used. Membranes arecurrently under development which will enable separation of water fromethanol; these are reported to be nearly ready for commercialexploitation. Hence use of a membrane is another option available forseparating water from the ethanol rich stream.

Preferably the water content of the thus produced relatively dry ethanolis less than about 5 mol %, and preferably less than about 2 mol %.

The impure alkyl alkanoate feedstock may, for example, compriseliquefiable components of a reaction product mixture produced bydehydrogenation of ethanol. Such ethanol may have been produced byhydration of ethylene, by the Fischer Tropsch process, or byfermentation of a carbohydrate source, such as starch (for example, inthe form of a corn steep liquor) It may alternatively be a by-product ofanother industrial process. It may contain, besides ethanol, minoramounts of water as well as small amounts of impurities resulting fromby-product formation during its synthesis. If there is provision forrecycle of recovered ethanol, then any by-products formed duringproduction of ethyl acetate will contribute to the level of by-productspresent in the feedstock. Impurities present in the ethanol feedstockmay include, for example, higher alcohols such as n-propanol,iso-propanol, n-butanol and sec-pentanol; ethers, such as diethyl ether,and di-iso-propyl ether; esters, such as iso-propyl acetate, sec-butylacetate and ethyl butyrate; and ketones, such as acetone, butan-2-one,and 2-pentanone. At least some of these impurities can be difficult toremove from ethyl acetate, even when they are present in quantities aslow a about 0.1 mol % or less, by traditional distillation proceduresbecause they have boiling points which are close to that of ethylacetate and/or form distillates therewith.

In the dehydrogenation step ethanol can be converted to ethyl acetate bya dehydrogenation procedure which comprises contacting a vaporousmixture containing ethanol and hydrogen with a dehydrogenation catalystin a dehydrogenation zone maintained under dehydrogenation conditionseffective for dehydrogenation of ethanol to yield ethyl acetate.

Typical dehydrogenation conditions include use of an ethanol:hydrogenmolar ratio of from about 1:10 to about 10000:1, a combined partialpressure of ethanol and hydrogen of up to about 50 bar (5×10⁶ Pa), and atemperature in the range of from about 100° C. to about 260° C.

Preferably the combined partial pressure of ethanol and hydrogen rangesfrom about 3 bar (3×10⁵ Pa) up to about 50 bar (5×10⁶ Pa), and is morepreferably at least 6 bar (6×10⁵ Pa) up to about 30 bar (3×10⁶ Pa), andeven more preferably in the range of from about 10 bar (10⁶ Pa) up toabout 20 bar (2×10⁶ Pa), for example about 12 bar (1.2×10⁶ Pa).

Dehydrogenation is preferably conducted in the dehydrogenation zone at atemperature of from about 200° C. to about 250° C., preferably at atemperature in the range of from about 210° C. to about 240° C., evenmore preferably at a temperature of about 220° C.

The ethanol:hydrogen molar ratio in the vaporous mixture fed intocontact with the dehydrogenation catalyst usually will not exceed about400:1 or about 500:1 and may be no more than about 50:1.

The dehydrogenation catalyst is desirably a catalyst containing copper,optionally in combination with chromium, manganese, aluminum, zinc,nickel or a combination of two or more of these metals, such as acopper, manganese and aluminium containing catalyst. Preferred catalystscomprise, before reduction, copper oxide on alumina, an example of whichis the catalyst sold by Mallinckrodt Specialty Chemicals, Inc., underthe designation E408Tu, a catalyst which contains 8% by weight ofalumina. Other preferred catalysts include chromium promoted coppercatalysts available under the designations PG85/1 (Kvaerner ProcessTechnology Limited) and CU0203T (Engelhard), manganese promoted coppercatalysts sold under the designation T4489 (Süd Chemie AG), andsupported copper catalysts sold under the designation D-32-J (Süd ChemieAG). E408Tu is a particularly preferred dehydrogenation catalyst.

In the dehydrogenation step the rate of supply of the ethanol feedstockto the dehydrogenation zone typically corresponds to an ethanol liquidhourly space velocity (LHSV) of from about 0.5 hr⁻¹ to about 1.0 hr⁻¹.

Hydrogen is produced as a result of the dehydrogenation reaction and canbe recycled to the dehydrogenation zone from downstream in the process.The hydrogen can be substantially pure hydrogen or can be in the form ofa mixture with other gases that are inert to the ethanol feedstock andto the dehydrogenation catalyst. Examples of such other gases includeinert gases such as nitrogen, methane and argon.

In the dehydrogenation zone, side reactions may also occur, includingformation of water. It is postulated that such side reactions, in thecase of production of ethyl acetate, include formation of acetaldehydewhich in turn can undergo aldol formation, followed by dehydration toform an unsaturated alcohol and water. These reactions can be summarisedthus:

CH₃CH₂OH=CH₃CHO+H₂  (5)

2CH₃CHO=CH₃CH(OH)CH₂CHO  (6) and

CH₃CH(OH)CH₂CHO=CH₃CH=CHCHO+H₂O  (7).

The crotonaldehyde produced by equation (7) can then undergohydrogenation to form n-butanol thus:

CH₃CH=CHCHO+H₂=CH₃CH₂CH₂CH₂HO.  (8)

Other side reactions which release water as a by-product includeformation of ketones, such as acetone and butan-2-one, and formation ofethers, such as diethyl ether.

In such a dehydrogenation process there is recovered from the ethylacetate production zone an intermediate reaction product mixturecomprising hydrogen and liquefiable products comprising ethyl acetate,ethanol, hydrogen and by-products containing reactive carbonyl groups;this intermediate reaction product mixture can be used as impure feed tothe recovery process of the invention. The step of recovering thisintermediate reaction product mixture can be effected in any convenientmanner and may include a condensation step in order to condenseliquefiable products present in the intermediate reaction productmixture. Alternatively the intermediate reaction product can be passeddirectly to step (b) without any intermediate condensation step.

The production of a relatively dry ethanol stream has been mentionedabove. This can be recycled, if desired, to the dehydrogenation step, ifused, or can be used for any other desired purpose.

In order that the invention may be clearly understood and readilycarried into effect, a preferred form of plant for the production ofethyl acetate, and a process in accordance with the invention will nowbe described, by way of example only, with reference to the accompanyingdrawings, wherein:

FIG. 1 is a flow diagram of a plant for the production of ethyl acetateconstructed to operate a process in accordance with the invention;

FIGS. 2 and 3 are triangular diagrams illustrating the boiling behaviourof ternary mixtures of ethanol, water and ethyl acetate at two differentpressures.

Referring to FIG. 1 of the drawings, it will be appreciated by thoseskilled in the art that, since the drawing is diagrammatic, manyconventional items of equipment, such as pumps, surge drums, flashdrums, heat exchangers, temperature controllers, pressure controllers,holding tanks, temperature gauges, pressure gauges, and the like, whichwould be required in an operating plant, have been omitted for the sakeof simplicity. Such items of equipment would be incorporated in anactual plant in accordance with standard chemical engineering practiceand form no part of the present invention. Moreover there are many waysof effecting heat exchange and the depiction of separate heat exchangerseach with its own heating or cooling line does not necessarily mean thatsingle heat exchanger units are necessary. Indeed in many cases it maybe more practicable and economic to use two separate heat exchangersinstead of one with a step change in temperature occurring in each. Itis also practicable to use conventional heat recovery techniques so asto recover heat from, or to increase the temperature of, one stream byheat exchange with another stream of the plant.

In the plant of FIG. 1 a stream of crude ethanol is pumped to the plantfrom a suitable holding tank (not shown) in line 1 at a pressure of 16.2bar absolute (16.2×10⁵ Pa) and at a temperature of approximately 30° C.and is admixed with recycled material from line 2. The resulting mixturein line 3 is heated by means of heat exchanger 4 to a temperature of166° C. thereby forming a vaporous stream which passes on in line to bemixed with a stream of hydrogen from line 6. The resulting mixturepasses on in line 7, is superheated in superheater 8 using high pressuresteam, and exits it in line 9 at a pressure of 14.8 bar absolute(14.8×10⁵ Pa) and at a temperature of 235° C. Line 9 leads to a firstdehydrogenation reactor 10 which contains a charge of a reduced copteroxide catalyst. A suitable catalyst is that sold under the designationE408Tu by Mallinckrodt Specialty Chemicals, Inc. In passage throughfirst dehydrogenation reactor 10 the mixture of ethanol and hydrogen ispartly converted by dehydrogenation according to equation (3) above toform ethyl acetate. This dehydrogenation reaction is endothermic.

The first intermediate dehydrogenation mixture exits reactor 10 in line11 at a temperature in the range of from 205° C. to 220° C. and isreheated in heater 12 under the influence of high pressure steam. Thereheated mixture flows on in line 13 to a second dehydrogenation reactor14 which also contains a charge of the same dehydrogenation catalyst asthat in reactor 10. Further dehydrogenation of ethanol to ethyl acetateoccurs in passage through second dehydrogenation reactor 14.

A second intermediate dehydrogenation mixture containing ethyl acetate,unreacted ethanol and hydrogen exits reactor 14 in line and is reheatedin reheater 16 which is heated by means of high pressure steam. Thereheated stream flows on in line 17 to a third dehydrogenation reactor18 which contains a charge of the same dehydrogenation catalyst as ispresent in reactors and 14.

The resulting third intermediate reaction mixture flows on in line 19 toheat exchanger which is also heated by means of high pressure steam. Thereheated mixture passes on in line 21 to fourth dehydrogenation reactor22 which contains a further charge of the same dehydrogenation catalystthat is loaded into the first, second and third dehydrogenation reactors10, 14, and 18.

A crude product mixture exits fourth dehydrogeration reactor 22 in line23, is cooled in passage through a heat exchanger 24, and emerges inline 25 at a temperature of 60° C. and at a pressure of 11.3 bar(11.3×10⁵ Pa) absolute.

The crude product mixture in line 25 comprises hydrogen, ethyl acetate,unconverted ethanol, water and minor amounts of impurities presenteither from contamination in the feed or recycle streams or from sidereactions in reactors 10, 14, 18 and 22. Examples of these impuritiesinclude iso-propanol, acetaldehyde, diethyl ether, methanol, acetone,di-iso-propyl ether, n-butyraldehyde, butan-2-one, sec-butanol,iso-propyl acetate, pentan-2-one, n-butanol, sec-pentanol, sec-butylacetate, ethyl butyrate, n-butyl acetate and di-n-butyl ether. Ofparticular significance in relation to this invention are thoseimpurities whose boiling points are close to that of ethyl acetate orwhich form azeotropic mixtures with ethyl acetate. These includeethanol, as well as certain carbonyl-containing compounds such asacetone, acetaldehyde and butan-2-one.

The crude mixture in line 25 flows into a knockout pot 26 which isprovided with a condenser (not shown) supplied with chilled coolant. Theuncondensed gases, which are now at a temperature of −10° C., arerecovered in line 27. A part of these gases is recycled in line 28 andcompressed by means of gas recycle compressor 29 to a pressure of 15.5bar (1.55×10⁶ Pa) absolute to form the gas stream in line 6 for supplyto the first dehydrogenation reactor 10. Another part is taken in line30 for a purpose which will be described hereunder. A purge stream istaken in line 31.

The condensate is removed from knockout pot 26 in line 32 and is pumpedby a pump (not shown) to heat exchanger 33. The resulting re-heatedliquid, now at a temperature of 60° C. to 80° C., is fed via line 34 andmixed with a hydrogen-containing gas which is at a temperature of 119°C. and has been compressed by a second gas compressor 35 to a pressureof 43.1 bar (4.31×10⁶ Pa) absolute so as to pass along line 36. Theresulting mixture flows on in line 37 into a reactor 38 which contains acharge of a selective hydrogenation catalyst which is chosen so asselectively to hydrogenate reactive carbonyl-containing compounds, suchas n-butyraldehyde, butan-2-one and the like, to the respectivecorresponding alcohols but not to effect any significant hydrogenationof ethyl acetate to ethanol. The inlet temperature to reactor 37 isadjusted as necessary to a temperature in the range of from 60° C. to80° C. in dependance upon the degree of deactivation of the catalyst butis chosen to be as low as possible consistent with obtaining anacceptable reaction rate because the equilibrium is favorable at lowertemperatures than at high temperatures. A preferred catalyst is 5%ruthenium on carbon available from Engelhard.

The resulting selectively hydrogenated reaction product is nowessentially free from reactive carbonyl compounds, such as aldehydes andketones, and exits reactor 38, in admixture with unreacted hydrogen, inline 39 at a, temperature of 70° C. to 90° C. This line leads to a lowerpart of a first distillation column 40 which is maintained at a pressureof 1.5 bar (1×10⁵ Pa) absolute. A bottoms product is withdrawn fromdistillation column 40 in line 41. Part of this is recycled todistillation column through line 42, column reboiler 43 and line 44. Theremainder is passed by way of line 45 to a purification section (orwater removal package) 46 in which it is treated in any convenientmanner for the removal of water (and possibly other impurities)therefrom so as to yield a stream of moderately dry ethanol for recycleto the first dehydrogenation reactor 10 by way of line 2. The precisedesign of water removal package 46 will depend upon the composition ofthe ethanol feed stream in line 1. The bottoms product in line 41typically comprises mainly ethanol with minor amounts of, for example,iso-propanol, water, C₄₊ alkanols, and traces or ketones, other estersand ethers.

An overhead stream, which typically comprises a major proportion ofdiethyl ether and lesser amounts of other ethers, methanol, ethanol,n-butyraldehyde, and alkanes, as well as traces of acetaldehyde, ethylacetate, and water, is recovered in line 47 and condensed by means ofcondenser 48. Uncondensed gases are purged in line 49, while theresulting condensate is recycled to the top of distillation column 40 asa reflux stream in line 50. A side draw stream is taken fromdistillation column 40 in line 51 and pumped by a pump (not shown) to asecond distillation column 52 which is maintained at an overheadpressure of 12 bar (1.2×10⁶ Pa) absolute.

From the bottom of distillation column 52 a stream comprisingsubstantially pure ethyl acetate is recovered in line 53, part of whichis recycled to a lower part of distillation column 52 by way of line 54,column reboiler 55, and line 56. The remainder forms the product streamin line 57 from the plant; this can be taken to storage or furtherdistilled in one or more further distillation columns, if desired, inorder to remove minor amounts of iso-propyl acetate, di-propyl ether,and 1-ethoxybutane.

An overhead product consisting mainly of ethanol, ethyl acetate andwater, besides smaller amounts of 1-ethoxybutane, methanol, diethylether and di-propyl ether and traces of alkanes, is taken in line 58 andcondensed by means of condenser 59. The resulting condensate passes onin line 60, some being recycled to the first distillation column by wayof line 61 while the remainder is recycled as a reflux stream to thesecond distillation column 52 in line 62. Reference numeral 63 indicatesa line for recovery of water and other materials from water removalpackage 46.

The compositions in mol % of some of the more important streams in theplant of FIG. 1 are set out in Table 2 below.

TABLE 2 Stream 1 2 9 25 27 32 37 39 45 49 51 57 61 63 Hydrogen 0.00 0.001.96 32.43 95.67 0.24 5.32 3.26 0.00 64.41 0.00 0.00 0.00 0.00 Carbon0.00 0.00 0.01 0.17 0.49 0.00 0.03 0.03 0.00 0.64 0.00 0.00 0.00 0.00monoxide Water 0.13 0.13 0.13 1.20 0.04 1.80 1.71 1.73 2.26 0.93 3.940.00 5.36 39.80 Methanol 0.01 0.00 0.01 0.01 0.00 0.01 0.01 0.01 0.000.20 0.06 0.00 0.09 0.00 Ethanol 99.84 99.84 97.82 49.25 1.39 73.5069.67 72.70 96.52 16.76 50.42 0.02 68.73 37.90 Ethyl acetate 0.00 0.000.01 15.03 0.91 22.32 21.18 20.86 0.00 7.17 45.40 99.98 25.57 0.00Acetaldehyde 0.00 0.00 0.00 0.51 0.03 0.75 0.71 0.01 0.00 0.13 0.14 0.000.19 0.00 Ethane 0.00 0.00 0.00 0.09 0.20 0.03 0.04 0.04 0.00 0.82 0.000.00 0.00 0.00 Methane 0.00 0.00 0.03 0.41 1.17 0.03 0.09 0.09 0.00 1.780.00 0.00 0.00 0.00 Di-ethyl ether 0.01 0.00 0.01 0.27 0.09 0.37 0.350.36 0.00 7.09 0.04 0.00 0.06 0.00 n-butyr- 0.00 0.00 0.00 0.01 0.000.01 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 aldehyde n-butanol 0.000.01 0.00 0.12 0.00 0.18 0.17 0.19 0.25 0.01 0.00 0.00 0.00 4.53sec-butanol 0.00 0.01 0.00 0.26 0.00 0.38 0.36 0.51 0.67 0.05 0.00 0.000.00 12.15 Butan-2-one 0.01 0.00 0.01 0.10 0.01 0.14 0.14 0.00 0.00 0.000.00 0.00 0.00 0.00 n-butyl acetate 0.00 0.00 0.00 0.05 0.00 0.08 0.070.07 0.10 0.01 0.00 0.00 0.00 1.81 sec-butyl 0.00 0.00 0.00 0.02 0.000.03 0.03 0.03 0.04 0.00 0.00 0.00 0.00 0.73 acetate Ethyl butyrate 0.000.00 0.00 0.04 0.00 0.07 0.06 0.06 0.09 0.00 0.00 0.00 0.00 1.63Di-butyl ether 0.00 0.00 0.00 0.00 0.00 0.01 0.01 0.01 0.01 0.00 0.000.00 0.00 0.18 n-hexanol 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.010.00 0.00 0.00 0.00 0.18 iso-butanol 0.00 0.01 0.01 0.01 0.00 0.01 0.010.01 0.01 0.00 0.00 0.00 0.00 0.18 Others 0.00 0.00 0.00 0.02 0.00 0.040.03 0.03 0.04 0.00 0.00 0.00 0.00 0.91 Total 100.00 100.00 100.00100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00100.00

FIG. 2 is a triangular diagram illustrating the distillationcharacteristics of mixtures of ethanol, water and ethyl acetate at 760mm Hg (1.01×10⁶ Pa) in which are plotted distillation lines fordifferent mixtures of the three components. FIG.3 is a similar diagramillustrating the distillation characteristics of the same ternary systemat 9308 mm Hg (12.41×10⁶ Pa ) It will be noted that there aresignificant differences between the distillation lines observed atdifferent operating pressures. In FIG. 2 the composition of a typicalfeed as might be supplied in line 39 of the plant of FIG. 1 is indicatedby point A. Point B indicates the composition of the side draw stream inline 51 for this feed. Point C indicates the composition of theresulting bottom stream in line 41 and point D indicates the compositionof the stream in line 61. The effective feed composition to column 40lies on the intersection of the straight line joining A and D with thestraight line joining points B and C. In FIG. 3 the points B and Drepresents the same compositions as the corresponding points in thetriangular diagram of FIG. 2. Point E represents the composition of thesubstantially pure ethyl acetate recovered in line 45.

The invention is further described in the following

EXAMPLES Examples 1 to 5

These Examples investigated the dehydrogenation of ethanol to ethylacetate in the presence of hydrogen. The apparatus used included adehydrogenation reactor made of stainless steel tubing which contained acharge of reduced copper oxide catalyst and which was immersed in a hotoil bath for heating purposes.

At start-up a charge of 200 ml of a tabulated copper oxide catalystavailable under the designation E408Tu from Mallinckrodt SpecialtyChemicals was placed in the reactor which was then purged with nitrogenat 14.5 bar (14.5×10⁵ Pa) A dilute H₂ in N₂ gaseous mixture at 3 bar(3×10⁵ Pa ) was passed over the catalyst at a rate of 600 standardliters per hour for 60 hours in order to effect catalyst reduction. Theoil bath was raised to the temperature indicated in Table 3 below. Thegas feed was then changed to pure hydrogen.

In operation hydrogen was introduced to the dehydrogenation reactor atrate of 2 standard liters per hour by way of a pressure regulator andflow controller through a line which was immersed in the bottom of theoil bath. An ethanol stream whose composition is set out in Table 3 wasfed as a liquid at a rate of 200 ml/hr to a vaporiser and mixed with thehydrogen. The resulting vaporous mixture of ethanol and hydrogen wassupplied to the dehydrogenation reactor.

The reaction products were cooled and the liquid condensate was analysedby gas chromatography. The results obtained are summarised in Table 3.

TABLE 3 Example No Feed 1 2 3 4 5 Temperature — 225 224 224 223 224 (°C.) Pressure — 4.53 2.74 7.91 28.6 47.0 (bar) (10⁵ Pa) Product Analysis(wt %) Acetaldehyde 0.007 2.578 5.317 1.388 0.114 0.027 Methanol 0.0640.063 0.087 0.034 0.013 0.011 Di-ethyl ether 0.108 0.133 0.120 0.1390.167 0.185 Ethanol 95.093 63.184 66.778 64.050 67.236 72.676 Acetone0.007 2.264 2.883 1.679 0.630 0.326 iso-propanol 3.403 1.582 1.081 2.1143.210 3.511 Di-iso-propyl 0.116 0.139 0.134 0.138 0.136 0.138 ethern-butyr- 0 0.012 0.010 0.006 0.004 0.005 aldehyde Ethyl acetate 0.03025.605 18.935 27.087 26.377 21.107 Butan-2-one 0.005 1.230 1.655 0.6610.074 0.015 sec-butanol 0.004 0.768 0.543 0.761 0.360 0.174 iso-propyl 00.184 0.144 0.040 0.316 0.318 acetate Pentan-2-one 0 0.316 0.309 0.2330.055 0.010 n-butanol 0.097 0.329 0.410 0.274 0.203 0.431 sec-pentanol 00.138 0.075 0.180 0.148 0.087 sec-butyl 0 0.058 0.037 0.057 0.052 0.044acetate Ethyl butyrate 0 0.132 0.115 0.093 0.030 0.075 n-butyl acetate 00.123 0.096 0.086 0.022 0.076 Water 0.540 0.789 0.920 0.660 0.450 0.460Others 0.526 0.373 0.351 0.320 0.403 0.324 Total 100.00 100.00 100.00100.00 100.00 100.00

Examples 6 to 9

In these Examples the selective hydrogenation of reactive carbonylcompounds in the presence of ethyl acetate was investigated using ahydrogenation reactor constructed out of stainless steel which wasimmersed in a hot oil bath for heating purposes.

In operation hydrogen was introduced by way of a pressure regulator andflow controller to the reactor which contained a charge of an Englehard5% ruthenium on carbon granular catalyst.

At start up a charge of 100 ml of the granular catalyst was placed inthe reactor which was then supplied with hydrogen at a pressure of 7.9bar (7.9×10⁵ Pa ), and warmed to 180-200° C. from room temperature at arate of 20° C. per hour. The reactor was held at 180-200° C. for onehour and then cooled. At the end of this procedure the catalyst wasfully reduced.

Dehydrogenation reaction product mixture whose composition is set outunder “Feed” in Table 4 was introduced to a heater at a rate of 130ml/hr and admixed with 7.8 standard liters per hour of hydrogen prior toadmission to the selective hydrogenation reactor. The reaction productwas cooled and the liquid condensate was analysed by gas chromatography.The results are summarised in Table 4.

TABLE 4 Example No Feed 6 7 8 9 Reactor Temperature — 91 80 72 110 (°C.) Pressure (bar) (10⁵ Pa) — 14.2 14.2 14.4 14.1 Product Analysis (Wt%) Acetaldehyde 0.904 0.034 0.040 0.038 0.039 Diethyl ether 0.579 0.4280.418 0.417 0.419 Ethanol 68.223 70.040 70.121 70.163 70.301 Acetone2.282 trace trace trace trace iso-propanol 1.004 3.232 3.233 3.213 3.231Di-iso-propyl ether 0.003 0.098 0.097 0.097 0.097 n-butyraldehyde 0.010trace trace trace trace Ethyl acetate 23.263 22.572 22.464 22.437 22.396Butan-2-one 0.170 0.002 0.004 0.007 0.003 sec-butanol 0.371 0.567 0.5660.560 0.567 iso-propyl acetate 0.186 0.185 0.184 0.184 0.184 n-butanol0.507 0.730 0.770 0.776 0.570 Water 1.410 1.170 1.170 1.200 1.270 Others1.088 0.942 0.933 0.908 0.923 Total 100.00 100.00 100.00 100.00 100.00Notes: The increased amount of n-butanol noted in Examples 6 to 9compared with the amount in the feed can be ascribed not only ton-butanol formed by hydrogenation of n-butyraldehyde present in the feed(the amount of which is, in any case, difficult to measure) but alsofrom hydrogenation of other products which contain C₄ groups and whichare included in the figure given for “others” in the feed.

Examples 10 to 12

The general procedure of Examples 6 to 9 was repeated using a differentfeed and different reaction conditions. The results are set out in Table5 below.

TABLE 5 Example No Feed 10 11 12 Reactor Temperature (° C.) — 79 98 119Pressure (bar) (10⁵ Pa) — 42.6 42.1 42.5 Product Analysis (Wt %)Acetaldehyde 0.952 0.006 0.006 0.006 Diethyl ether 0.030 0.030 0.0290.033 Ethanol 64.703 65.930 66.034 65.627 Acetone trace 0 0 0iso-propanol 0.022 0.032 0.035 0.038 n-butyraldehyde trace 0 0 0 Ethylacetate 31.692 31.410 31.155 31.409 Butan-2-one 0.301 trace trace 0.001sec-butanol 0.487 0.803 0.806 0.810 n-butanol 0.560 0.588 0.596 0.573Water 0.620 0.600 0.700 0.890 Others 0.633 0.601 0.639 0.613 Total100.00 100.00 100.00 100.00

Example 13

A mixture containing ethanol, water, ethyl acetate and other componentswas distilled in a continuous feed laboratory distillation apparatushaving the general layout of columns 40 and 52 of FIG. 1, except thatline 51 received condensate from line 50, rather than a side draw streamfrom an outlet positioned somewhat lower in column 40. A bleed ofO₂-free nitrogen was supplied to column 40 so as to ensure that oxygenwas excluded from column 40 in order to prevent oxidation of anyoxygen-sensitive components in the feed in line 39 such as aldehydes.Hence column 40 was operated at a few milliards over atmosphericpressure. The feed to column was vaporiser in a stream of O₂-freenitrogen prior to introduction into column 40. The reflux temperature incolumn 40 was 64° C., the overhead temperature was 72° C. and thetemperature at the bottom of the column was 73° C. The reflux ratio was5:1. The operating pressure in column 52 was 12.4 bar (1.24×10⁶ Pagauge). The overhead temperature was 160° C., the reflux temperature was153° C. and the boiler temperature was 204° C. The reflux ratio was2.8:1. The distillation column had 3 thermocouples positioned near thetop, at the mid point and near the bottom, the readings of which were163° C., 180° C. and 180° C. respectively. The results obtained arelisted in Table 6 in which amounts are in % by weight.

TABLE 6 Line No. 39 51 41 61 53 Acetaldehyde 0.009 0.007 0.013 0.446Methanol 0.090 0.141 0.199 Diethyl ether 0.073 0.113 0.226 Ethanol57.626 31.077 96.579 71.382 0.064 iso-propanol 0.027 0.087 Ethyl acetate40.514 68.021 0.018 24.811 99.890 Butan-2-ol 0.548 1.499 n-butanol 0.1920.021 0.519 0.010 Ethyl butyrate 0.117 0.307 Butyl acetate 0.136 0.358Water 0.550 0.590 0.330 2.920 0.010 “Light” unknowns 0.020 0.029 0.003“Heavy” unknowns 0.098 0.001 0.290 0.013 0.026 Total 100.00 100.00100.00 100.00 100.00

What is claimed is:
 1. A process for the purification of an impurefeedstock comprising an alkyl alkanoate which contains not more than 12carbon atoms, the process comprising: (a) providing an impure feedstockcontaining an alkyl alkanoate which contains not more than 12 carbonatoms and at least one of an alkanol and water, said feedstock furthercontaining at least one impurity selected from the group consisting ofaldehydes and ketones which contain the same number of carbon atoms assaid alkyl alkanoate; (b) contacting said impure feedstock with aselective hydrogenation catalyst in the presence of hydrogen in aselective hydrogenation zone maintained under selective hydrogenationconditions effective for selective hydrogenation of said impuritiesthereby to hydrogenate said impurities to the corresponding alcohols;(c) recovering from the selective hydrogenation zone a selectivelyhydrogenated reaction product mixture comprising said alkyl alkanoate,hydrogen, and said corresponding alcohols; (d) distilling material ofthe selectively hydrogenated reaction product mixture in at least onedistillation zone so as to produce purified alkyl alkanoate therefrom;and (e) recovering said purified alkyl alkanoate.
 2. A process accordingto claim 1, in which the impure feedstock comprises a reaction productobtained by converting an alkanol to said alkyl alkanoate by a procedureselected from the group consisting of: (i) dehydrogenation, (ii)oxidation, (iii) reaction with an aldehyde, and (iv) oxidation to thecorresponding aldehyde followed by the Tischenko reaction.
 3. A processaccording to claim 1, in which said alkyl alkanoate is a C₂₊ alkyl C₂₊alkanoate.
 4. A process according to claim 1, in which said alkylalkanoate is selected from the group consisting of ethyl acetate,n-propyl propionate, and n-butyl butyrate.
 5. A process according toclaim 1, in which said alkyl alkanoate is ethyl acetate.
 6. A processaccording to claim 1, in which the selective hydrogenation conditions ofstep (b) include use of a feedstock:hydrogen molar ratio of from about1000:1 to about 1:1, a combined partial pressure of feedstock andhydrogen of from about 5 bar (5×10⁵ Pa ) to about 80 bar (8×10⁶ Pa ),and a temperature in the range of from about 40° C. to about 120° C. 7.A process according to claim 6, in which the combined partial pressureof feedstock and hydrogen in step (b) is from about bar 25 (2.5×10⁶ Pa )to about 50 bar (5×10⁶ Pa).
 8. A process according to claim 1, in whichthe selective hydrogenation catalyst comprises a metal selected from thegroup consisting of nickel, palladium, platinum, ruthenium, rhodium andrhenium.
 9. A process according to claim 8, in which the catalystcomprises ruthenium on carbon.
 10. A process according to claim 1, inwhich the rate of supply of impure feedstock to the selectivehydrogenation zone corresponds to a liquid hourly space velocity (LHSV)of from about 0.1 hr⁻¹ to about 2.0 hr⁻¹.
 11. A process according toclaim 1, in which the impure feedstock is an impure ethyl acetatefeedstock which contains, in addition to ethyl acetate and impurities,also water and ethanol and in which step (d) comprises supplyingmaterial of the selectively hydrogenated reaction product mixture to afirst distillation zone maintained under distillation conditionseffective for distillation therefrom of a first distillate comprisingethyl acetate, ethanol and water, recovering a first distillatecomprising ethyl acetate, ethanol, and water from the first distillationzone and a bottom product comprising ethanol and water, supplyingmaterial of the first distillate to a second distillation zonemaintained under distillation conditions effective for distillationtherefrom of a second distillate comprising ethanol, water, and ethylacetate and so as to yield a purified ethyl acetate bottom product, andrecovering a purified ethyl acetate bottom product from the seconddistillation zone.
 12. A process according to claim 11, in which thefirst distillation zone is operated at a pressure of less than about 4bar (4×10⁵ Pa ).
 13. A process according to claim 11, in which the firstdistillation zone is operated at a pressure of from about 1 bar (10⁵ Pa) to about 2 bar (2×10⁵ Pa ).
 14. A process according to claim 11, inwhich the second distillation zone is operated at a pressure of fromabout 4 bar (4×10⁵ Pa ) to about 25 bar (2.5×10⁶ Pa ).
 15. A processaccording to claim 11, in which the second distillation zone is operatedat a pressure of from about 9 bar (9×10⁵ Pa ) to about 15 bar (15×10⁵ Pa).
 16. A process according to claim 11, in which an ethanol rich streamis recovered from the bottom of the first distillation zone, while anoverhead stream that contains light components having lower boilingpoints than ethyl acetate and its azeotropes with water and ethanolpresent in the selectively hydrogenated reaction product mixture isrecovered from the first distillation zone, and in which the firstdistillate comprises a liquid draw stream which is recovered an from anupper region of the first distillation zone and which comprises ethylacetate, ethanol, and water.
 17. A process according to claim 16, inwhich the ethanol rich stream recovered from the bottom of the firstdistillation zone is subjected to treatment for the removal of watertherefrom.
 18. A process according to claim 16, in which the firstdistillate contains from about 40 mol % to about 55 mol % ethyl acetate,from about 1 mol to about 6 mol % water, not more than about 1 mol %other components, and the balance ethanol.
 19. A process according toclaim 16, in which the first distillate is passed to the seconddistillation zone which is operated at a pressure of from about 9 bar(9×10⁵ Pa) absolute to about 15 bar (1.5×10⁶ Pa ) absolute.
 20. Aprocess according to claim 16, in which the second distillate isrecovered as an overhead stream from the second distillation zone, whilea bottom product comprising purified ethyl acetate is recovered from thesecond distillation zone, the second distillate being returned to thefirst distillation zone at a point above the feed point of the materialof the selectively hydrogenated reaction product mixture.
 21. A processaccording to claim 20, in which the bottom product from the seconddistillation zone contains from about 99.8 mol % to about 99.95 mol %ethyl acetate.
 22. A process according to claim 1, in which step (d)comprises extractive distillation with an extractive agent comprisingpolyethylene glycol and at least one material selected from the groupconsisting of dipropylene glycol, diethylene glycol, and triethyleneglycol.
 23. A process according to claim 1, in which step (d) comprisesextractive distillation in the presence of an extractive agentcontaining dimethyl sulfoxide.
 24. A process for the production of analkyl alkanoate containing not more than 12 carbon atoms bydehydrogenation of an alkanol, the process comprising: (i) contacting avaporous mixture containing an alkanol and hydrogen with adehydrogenation catalyst in a dehydrogenation zone maintained underdehydrogenation conditions effective for dehydrogenation of an alkanolto yield an alkyl alkanoate containing not more than 12 carbon atoms;(ii) recovering from the dehydrogenation zone an intermediate reactionmixture comprising hydrogen and liquefiable products comprising saidalkyl alkanoate, said alkanol, hydrogen and impurities selected from thegroup consisting of aldehydes and ketones which contain the same numberof carbon atoms as said alkyl alkanoate; (iii) contacting at least aportion of the liquefiable products of the intermediate reaction productmixture with a selective hydrogenation catalyst in the presence ofhydrogen in a selective hydrogenation zone maintained under selectivehydrogenation conditions effective for selective hydrogenation of saidimpurities thereby to hydrogenate said impurities to the correspondingalcohols; (iv) recovering from the selective hydrogenation zone aselectively hydrogenated reaction product mixture comprising said alkylalkanoate, hydrogen, and said corresponding alcohols; (v) distillingmaterial of the selectively hydrogenated reaction product mixture in atleast one distillation zone so as to produce purified alkyl alkanoatetherefrom; and (vi) recovering said purified alkyl alkanoate.
 25. Aprocess according to claim 24, wherein the dehydrogenation conditionsinclude use of an alkanol:hydrogen molar ratio of from about 1:10 toabout 1000:1, a combined partial pressure of alkanol and hydrogen offrom about 3 bar (3×10⁵ Pa ) up to about 50 bar (5×10⁶ Pa), and atemperature in the range of from about 100° C. to about 260° C.
 26. Aprocess according to claim 25, wherein the dehydrogenation conditionsinclude use of a combined partial pressure o alkanol and hydrogen of atleast about 6 bar (6×10⁵ Pa) up to about 30 bar (3×10⁶ Pa).
 27. Aprocess according to claim 24, in which the dehydrogenation conditionsinclude use of a temperature of between about 200° C. and about 250° C.28. A process according to claim 24, in which the dehydrogenationcatalyst is a copper containing catalyst which comprises, beforereduction, copper oxide on alumina.
 29. A process according to claim 24,in which the rate of supply of the feedstock to the dehydrogenation zonecorresponds to an alkanol liquid hourly space velocity (LHSV) of fromabout 0.5 hr⁻¹ to about 1.0 hr⁻¹.
 30. A process according to claim 24,in which the impure feedstock contains water and ethanol and in which instep (d) there is recovered an ethanol stream for recycle to thedehydrogenation zone.